Conventionally, a spatial distribution of a magnetic field (hereafter, also referred to as a “magnetic field distribution”) is utilized in various applications such as locating an abnormal electric current path in an electronic component and examining a disease part of a human body. A superconducting quantum interference device and a magnetoresistive sensor, for instance, are used as magnetic field sensors to measure such a magnetic field distribution. The superconducting quantum interference device is also referred to as a “SQUID element”.
Magnetic force microscopy (MFM) may also be used to obtain a magnetic field distribution. A magnetic field sensor which includes a sharpened silicon tip coated with a magnetic thin film is used for a MFM probe. Such a probe is also referred to as a “magnetic probe”. Patent Literature (PTL) 1 proposes a structure in which a carbon nanotube magnetic probe is used as a MFM probe. PTL 2 describes a method for measuring a three-dimensional distribution of, for instance, a magnetic field and an electric field in a three-dimensional space.
In the method described in PTL 2, the Laplace equation which is a fundamental equation of a static magnetic field is exactly solved using, as a boundary condition, a two-dimensional magnetic field distribution and a two-dimensional distribution of the gradient of the magnetic field obtained at a specific measurement plane, thus obtaining a three-dimensional magnetic field distribution in a space around the measurement plane. The gradient of the magnetic field mentioned here means a gradient of a magnetic field in a direction normal to the measurement plane. The space around the measurement plane includes both a three-dimensional space above the measurement plane and a three-dimensional space below the measurement plane.
With method described in PTL 2, a structure of a source of a magnetic field (magnetic field source) can be imaged using measurement data of a magnetic field distribution obtained in an area away from the magnetic field source. The image showing the structure of the magnetic field source can be utilized for medical diagnosis and electronic component failure analysis, for example.
When a magnetic field source in the space below a measurement plane is measured, an electronic circuit and a mechanical component for processing signals from a magnetic field sensor are typically present in the space above the measurement plane. These are not objects to be measured, but are the magnetic field sources. The method described in PTL 2 allows the distribution of a field to be precisely analyzed even when magnetic field sources are included in both of the spaces above and below the measurement plane.
The spatial resolution in the measurement of a magnetic field distribution depends on the size of a coil used in a SQUID element or the size of a magnetoresistive sensor. Miniaturizing such a magnetic field sensor allows the magnetic field distribution to be imaged with a higher spatial resolution. There is, however, actually a limit to the miniaturization of the magnetic field sensor. For example, it is hard to manufacture a magnetic field sensor of 100 nm or less in size. Besides, a miniaturized magnetic field sensor outputs a small electric signal from a sensor sensing area thereof, and suffers a low signal to noise ratio (S/N).
To address this, a method of rotating a magnetic field sensor is used as described in PTLs 3, 4, and 5. For example, when an X direction and a Y direction are perpendicular to each other, if a magnetic field sensor has a size that is greater in the X direction and smaller in the Y direction, the resolution of a magnetic field distribution is lower in the X direction and higher in they direction. Consequently, the rotation of the magnetic field sensor increases the resolution of a magnetic field distribution in more directions.